Ah yes, thats what I was referring to as the "Michelle"
I disconnected my cart mod wires from pads 13-24 and the screen was still jumbled up like in the picture. I then disconnected 1-12 and the flashback works again. I guess I had some problem with my soldering technique or maybe there is a bad pin on the edge connector or something.
Not sure why this was moved to the Pinball Wizard forum when its mostly about the Flashback . . .
I got my Flashback cart mod to work. I had to de-solder all 24 wires and then put them back. I'm not sure what I did wrong the first time, but all is well now.
Finding the ground connection on the pads came down mainly to identifying other ground on the board and then testing for continuity between known ground and the unknown pads. In this case the board diagrams on Atariage showed me known ground.
Anyway, I've put pics with captions for my mod here: